Resistance element



Patented 20, 1939 UNITED" STATES PA ENT or-Flea.

v RESISTANCE ELEMENT Newton 0. Schellengel', Elkhart, Ind., assignor; to

Chicago, Telephone Supp y 00., a corporation of Indiana Elkhart, Ind., X

No Drawing. Application December 18, 1935,

Serial No. 55,005

6 .Claims.

This invention relates to a new and improved resistance element and more particularly to resistance elements comprising ingredients tending to stabilize the resistance value under varying conditions of temperature and moisture.

. The invention is particularly directed to resistance elements comprising a high resistance coating on a substantially non-conducting base. These resistances comprise bases of paper, fibre,

l0 Bakelite or other artificial resins and conductive.

coatings consisting of carbon in various forms, with difierent types of vehicles and. binders These coatings are usuallypainted or sprayed upon the bases. Some of them harden or cure merely on exposure to the air, while others ,re-

quire varying degrees of heat to drive. off volatile constituents and react or cure the binder. In modern radio circuits these resistances are used as tone and volume controls and are included in sensitive portions of the circuits. Design practice has tended to reduce the size of these controls so that the resistance strip is arcuate in form and usually of a diameter .of less than one and a half inches. The .units are also of a thickness of less than seven-sixteenths of an inch so that, in order to accommodate the contacts, contact arms and other structural elements, the resistance itself must be of slight thickness. N

Into these units are. placed resistance elements which frequently measure. as high as several.

=meg0hms. In addition, .the resistance gradient in. "nearly all cases must be built to very close pro gression tolerances and must accurately conform to such irregular curve forms as the radio set en'-. 33 gineer may specify for the proper control of a given radio set. I For such elements to give commercialservice they must be stable under varying operating conditions, particularly with reference to varying degrees of relative humidity.

Radio inspection departments often put such units throughsevere humidity tests and require a high "degree of stability of resistance throughout repeated cycles of a wide range of tempera- -ture and humidity conditions. Some controls now manufactured will show an increase in resistance above that at normal conditions, which reaches as high as when subjected to a relative humidity of at a temperature of 50 F. over a period of fifty hours. Some of the more stable types of resistance elements show aresistance increase ofconly from 8% to 20% 'above normal under similar humidity tests. Even these latter increases above normal are, in the 55 majority of cases, highly detrimental to. the func (cram-act tioning of the radio circuit in which they are used.

It is an object of the present invention to provide a new and improved resistance element which is stable under severe changes of temper: ature and humidity.

It :is'a further object to provide meanswhereby various types of resistance elements may be rendered stable under the conditions stated. v a

It is an additional object to provide methods for making and stabilizing resistance compounds and elements;

' It is also an object to provide means, methods and elements of this character which are com- I paratively low in cost, simple in use and adapted for commercial production.

Other and further objectswill appear as the same conditions may be added in-such propor-' tions as to directly counterbalance the trend of the resistance unit and make a combined unit agents in combination with various types of resistance compounds and have found thatthere' are many salts and a number oi. mild acids which may be eifectively usedfor this purpose; Among the salts I have' found sodium chloride to be particularly effective. Other neutral salts which may be used include sodium sulphate and ferric chloride, while the'basic salts, sodium carbonate and sodium tetraborate, are also suitable, as'is the double salt ferric ammonium sulphate. Mild inorganic acids, such as boric or arsenic acids, may be used or organic acids such' as tannic acid.

it to be simple and effective in use and it may be readily secured in sufficient purity at lowcost.

The sodium chloride may be added to or incorporated with the resistance element in several. ways, and the most infective way depends upon the character of the unit with whichitis used. I have found that the quantity required is relawhich is extremely stable under such conditions. I have used a number of different stabilizing I preferably use sodium chloride as I have found tively small and generally is somewhere between one-quarter of onepercent and four percent of the weightof the conductive pigments being employed.;' The proper amount for each .type .or combination of pigments, vehicles and bindersmust be determined by experiment. I

In one method of incorporating the stabilizing agent it is first reduced to a very fine powder in a ball mill. It is then introduced into a second ball mill' with the conductive pigments, which comprise finely divided carbon, and the mixture The mixture is sprayed onto .the bases to form a thin film. and is air dried at moderate'temperatures. In some cases sheet Bakelite is used for the bases and in other cases paper approximately .020". in thickness is used. Repeated tests show .that the stabilizing effect of. the salt is just aspermanent in the film on Bakelite as it is-when used on the more absorbent paper base or supporting member,

By the use of my stabilizer or neutralizing ingredient in the resistance element, I am able to hold the res'stance characteristics within a range of one to two percent when the element is subjected to the most adverse operating conditions, that is, high relative humidity and high temperature. In all cases the element returns to its normal condition when the moisture content is reduced to its original proportions.

.With resistance elements of a wholly carbonaceous character, I have found it is not generally desirable to rhix the stabilizing material with the pigments and binder before subjecting them to the high temperature used to cureor react the binder. This is not desirable when the necessary temperatures are such as to affect the stabilizing agent and thus impair its later. action as a stabilizer. In such cases I have found it desirable to take the salt in solution and spray it upon the surface of the resistor after the resistor has been through its heat treatment. The-minute cavities in the surface of the resistance unit hold the salt after the moisture has been driven oiI. Only a small loss in eflectiveness was found in such units after many cyclesof high humidity in the testing apparatus.

' With some types of resins when used as binders I for the resistance particles, such, for example, as

vinyl resin, I have found that the surface is not 1 sufiiiciently porous to effectively hold the stabilizing agent and in such cases the agent may be incorporated with the resistance compound.

Thus it will be seen that my stabilizing agent may be used in several ways, depending .upon conditions. It maybe incorporated with the resistance material or may be applied to the suri face of either the resistance carrying base or of the resistance material after the latter has .gone

v been found to be suitable.- The essentiafcharacteristics of the stabilizing agent are that itv reduce resistance under conditions oi. high humidity, that it be relatively inert toward theconstituents of the resistance element and the contacts, and that it be capable of repeated reversal in going through cycles of humidity without substantial loss in effectiveness.

Since the increase in resistance due to humidity varies with dififerent combinations of resistance materials and binders, the amount and character of the stabilizing material for use with different resistance units will vary and can best be determined by experiment.

While I have disclosed certain preferred embodiments of my invention, these are to be taken as' illustrative only, as it is capable of change and I variation to meet diifering conditions and requirements and I contemplate such modifications as come within the spirit and scope of the appended claims.

I claim: v I

1. In a resistance element, in combination, carbonaceous resistance material, a resinous binder for said material, the resstance of said material and binder increasing with increases in humidity, and a stabilizing agent associated therewith, said agent having a resistance which decreases with increases of humidity, said in- 01 eases and decreases in resistance substantially counterbalancing each other.

2. In a resistance element, in combination, car- 3. In a resistance element, in combination,

finely divided carbonaceous resistance material, a finely divided stabilizing agent intimately mixed with the resistance material, and a resinous binder for the material and agent, the stabilizing agent being reversely aflected as to conductivity by moisture to the eilect of moisture on other constituents whereby the resistance value of the constant quantity as to maintain the resistance of theunit substantially constant under fluctuating temperature and humidity conditions.

5. The method oi making a substantially stable resistance element, which comprises reducing sodium chloride to a fine powder, adding the sodiumchloride to carbonaceous material, adding a binder, treating the mixture to thoroughly mix the sodium chloride and carbonaceous material, and to finely divide them, and placinga thin layer of the mixture 'on a substantially noncond'ucting base.

6. The method of making, a substantially stable resistance element, which comprises reducing a stabilizing agent to arfine powder, adding the agent to carbonaceous material and a binder,

1 material on a substantially non-conducting base.

' NEWTON C. SCHELLENGER. 

